The most common reason for this issue is that your mail client fails to authenticate the server. Generally, this is caused when the email client’s (for example MS Outlook) SMTP authentication is not switched on.

This error can be resolved using the following:

1) POP before SMTP — check your email for new mail before trying to send an email. This adds your IP address to a relayhosts file and allows any email to be sent from that IP address for up to 30 minutes. After 30 minutes, you have to authenticate again.

2) Configure your email client to authenticate automatically by checking the “my server requires authentication” box in your email configuration. You will be able to send mail without having to “POP” your mail box first.

Another possible reason for the issue is an Antivirus or Firewall software that is checking your outgoing mail connections. If you have properly set your mail client, as explained above, and the issue still persists, turn off the antivirus and the firewall software you have installed on your local computer and try sending the e-mails again.